I think it really depends. I mean, if someone presented me with a gift card for Asda or Argos, I'd be like "eh what?" but if it was for somewhere I would actually shop - HMV, H&M, etc. - and allowed me to pick something that I really liked or wanted, then that's fair enough.
If you have no time and a gift card if your best option at the moment, just explain to her in a wee laughable letter that "I wanted to order your something specifically but it wouldn't have come in time - I hope you'll be able to get something that you really want with this" or whatever.